The present invention relates to variable speed constant frequency devices and methods for maximizing engine generator efficiency. In particular, the present invention relates to maximizing the efficiency of an engine generator using a multi-path variable speed constant frequency device.
Engine generators are typically used to provide power independent of that provided by a public utility. Engine generators may be used, for example, as primary power generators that provide a reliable source of power for industrial, commercial, and residential applications. For example, an industrial application such as a high value manufacturing process may require a reliable source of electricity to ensure that product development (e.g., semi-conductor fabrication) is not damaged by power fluctuations or power loss (e.g., caused by transmission line malfunction). A mining operation that is too remote to access a utility power grid may require an engine generator to provide power for day-to-day mining operations. A commercial application may include, for example, a communication facility (e.g., broadcasting, telephone, Internet, etc.) that requires an independent power source (e.g., engine generator). Thus facilities that are not readily accessible to a utility grid may rely on, for example, engine generators to provide continuous service to customers that rely on such facilities.
Engine generators generally include a fuel powered engine and a generator. The engine typically operates at a predetermined speed to drive a shaft, which causes a generator (e.g., synchronous generator) to generate power. The frequency of the power generated by the engine generator is typically dependent on engine speed. Thus, the faster the engine operates, the greater the frequency of the generated power. Because engine generators are typically used to provide a substantially constant frequency (e.g., 50/60 Hz), the engine continuously operates at a substantially constant speed.
FIG. 1 illustrates device 100, which is a conventional standard line connection that couples engine generator 110 to a load via switch 120. This standard line connection is particularly useful and efficient for providing power to constant, nonvarying loads. In addition, device 100 is useful if the engine operates at a substantially constant speed. Because effective device 100 operation is dependent on constant load and constant engine operating speed, there are several drawbacks to device 100.
One problem with device 100 is that it cannot modify or change the frequency of the power generated by the engine generator. That is, device 100 routes power to the load at the frequency generated by the engine generator. This limitation requires that the engine operate at the same speed to provide power at the frequency required by the load. Thus, when the load draws less power than the engine generator is capable of producing, the engine may burn excess fuel. Assuming that the engine has a specified power rating, the engine wastes fuel whenever the load requires substantially less power than that of the engine's power rating.
Another problem with device 100 is that the engine generator cannot maintain a constant frequency when the load experiences changes (step changes) in demand (e.g., caused by activating an air conditioner). To compensate for this deficiency, engines have to be dramatically oversized to handle step changes in load. If the engine is oversized, then it may be forced to generate power at a level much lower than it is capable of producing. Thus, maximum utility of the engine is not realized.
To rectify the problems associated with device 100, variable speed constant frequency (VSCF) devices have been developed. FIG. 2 illustrates VSCF device 200 that enables an engine generator to operate at various speeds and still be able to provide power at a constant frequency to the load. Device 200 includes rectifier 210, inverter 220, inductor 230, and capacitor 240. Engine generator 205 is coupled to rectifier 210 and inductor 230 is connected to the load. Optional battery 250 or other transient power source may be connected to inverter 220.
Rectifier 210 and inverter 220 (i.e., converter pair) operate in conjunction with each other such that the engine can operate at variable speeds and provide power at a constant frequency to the load. But the converter pair constantly consumes energy (e.g., exhibited as heat loss) regardless of whether the engine generator is producing the required frequency. Thus, when the engine generator is operating under full load, its fuel efficiency is less than that when device 100 of FIG. 1 is used to deliver the same power. VSCF device 200 provides fuel savings when fuel consumption decreases to a point where the energy saved by decreased engine speed outweighs the converter pair's energy dissipation.
The converter pair actively modifies the frequency of the generator output signal when the load experiences step changes. But sometimes the step change in load is too great for the engine and the VSCF devices to handle. For example, if an air conditioning unit turns on, the engine may experience a brief lull in power generation for which the VSCF device cannot adequately compensate.
VSCF device 200 overcomes the problem with load changes by providing optional battery 250 or other transient power source. Inverter 220 may draw power from battery 250 to maintain a constant frequency when the load is experiencing a step change. But battery 250 is typically bulky, toxic, requires extensive maintenance, and must be replaced frequently. In addition, the battery adds to the actual size of floor space required for engine generator, batteries, etc.
Moreover, VSCF device 200 is less reliable than VSCF device 100 because rectifier 210 or inverter 220 failure would prevent power delivery to the load.
VSCF devices enable engines to run at various speeds while providing power at a specified frequency. Typically, the engine is instructed to operate at the speed required to generate enough power to supply the load. Running the engine at this speed, however, may not be the most economical speed to conserve fuel, to maximize power density, or to minimize noise or emissions of the engine. The engine may be able to run at more optimal speeds, and still provide the required power. In fact, engines operating without a VSCF device often do not provide adequate power density because it has to operate at a speed less than its optimal RPM rating so that it can provide a desired constant frequency output.
In view of the foregoing, it is an object of this invention to provide a multi-path VSCF device that provides power to a load at a substantially constant frequency.
It is a further object of this invention to maximize fuel efficiency while supplying power to a load.
It is also an object of this invention to operate the engine at an optimal speed for any given load such that emissions and noise are minimized.
It is also an object of this invention to enable engines to operate at rated speed to provide sufficient power density and a desired frequency output.